Project spotlight - River Mersey banks on NAUE
NAUE’s Terrafix® B813 – a geosynthetic sand matting product – has been employed in the restoration of a stretch of eroded river bank along the northern side of the River Mersey near Warrington, Cheshire UK. The pathway, which had been severely affected by a bank slip following a rapid drop in water levels, affords views across the Mersey to the Woolston Eyes – an SSSI (Site of Special Scientific Interest) with a history dating back to the Middle Ages.
Part of a multi-million-pound River Mersey Flood Risk Management Scheme, restoration of the 370m section of river bank required construction of a 20m wide revetment, with a typical gradient of 1:3.5, followed by installation of a sheet piled wall along the top of the bank, to be finished with a timber coping and timber cladding on the dry face.
The engineered design required the revetment to be faced with rock reinforcement which would extend below water level to the river bed – providing physical support for the sheet piles and protecting the river bank from future scour. Due to the weak condition of the damaged river bank, the design first required placement of a geotextile layer to provide, stability, separation, filtration and protection.
Terrafix® B813 sand matting has a mass of around 6kg per square metre; allowing it to sink in water and contours to the substrate below, making it ideal for underwater bed stabilisation applications. The three-dimensional structure and flexibility makes it resistant to damage from the impact of bucket loads of stone cover, without loss of filtration performance. The ballast layer also prevents against water movement forming folds in the geotextile. The non-woven geotextile retains weak soils, while allowing the passage of water, and the large effective pore space of the Terrafix® structure provides an excellent environment to establish the propagation of local flora and fauna.
Access to the damaged bank was very restricted and installation of the Terrafix® B813 matting, and subsequent placement of rock armour, had to be carried out from the river via a floating pontoon using a long reach excavator fitted with a spreader bar; placing the material from the top of the river bank and deploying full rolls into the River Mersey.
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